PLANETARY SYSTEMS. 97 



most, exclusively to motion and mass, and but little to volume. 

 The absolute distance of a satellite from its central body is 

 greatest in the case of the outermost or seventh satellite of 

 Saturn, its distance from the body round vv^hich it revolves 

 amounting to more than two millions of miles, or ten times as 

 great a distance as that of our moon from the Earth. In the 

 case of Jupiter we find that the outermost or fourth attendant 

 moon is only 1,040,000 miles from that planet, while the dis- 

 tance between Uranus and its sixth satellite (if the latter real- 

 ly exist) amounts to as much as 1,360,000 miles. If we com- 

 pare, in each of these subordinate systems, the volume of the 

 main planet with the distance of the orbit of its most remote 

 satellite, we discover the existence of entirely new numerical 

 relations. The distances of the outermost satellites of Uranus, 

 Saturn, and Jupiter are, when expressed in semi-diameters 

 of the main planets, as 91, 64, and 27. The outermost satel- 

 lite of Saturn appears, therefore, to be removed only about 

 one fifteenth further from the center of that planet than our 

 moon is from the Earth. The first or innermost of Saturn's 

 satellites is nearer to its central body than any other of the 

 secondary planets, and presents, moreover, the only instance 

 of a period of revolution of less than twenty-four hours. Its 

 distance from the center of Saturn may, according to Madler 

 and Wilhelm Beer, be expressed as 2-47 semi-diameters of that 

 planet, or as 80,088 miles. Its distance from the surface of 

 the main planet is therefore 47,480 miles, and from the outer- 

 most edge of the ring only 4916 miles. The traveler may 

 form to himself an estimate of the smallness of this amount 

 by remembering the statement of an enterprising navigator, 

 Captain Beechey, that he had in three years passed over 72,800 

 miles. If, instead of absolute distances, we take the semi-di- 

 ameters of the principal planets, we shall find that even the 

 first or nearest of the moons of Jupiter (which is 26,000 miles 

 further removed from the center of that planet than our moon 

 is from that of the Earth) is only six semi-diameters of Jupiter 

 from its center, while our moon is removed from us fully 60 ^d 

 semi-diameters of the Earth. 



In the subordinate systems of satellites, we find that the 

 same laws of gravitation which regulate the revolutions of the 

 principal planets round the Sun hkewise govern the mutual 

 relations existing between these planets among one another 

 and with reference to their attendant satellites. The twelve 

 moons of Saturn, Jupiter, and the Earth all move like the 

 primary planets from west to east, and in elliptic orbits, do- 



Vol. I.— E 



